This invention relates generally to ultrasound imaging and more particularly to ultrasound transducers.
Ultrasound systems may be configured to use different types of transducers, such as one-dimensional (1D), 2D and/or 3D transducers. 1D transducers have a 1D array that can be arbitrarily steered and focused in the azimuth (in-plane) dimension. In the elevation (out-of-plane) dimension, the transducer typically has a fixed focus formed by using an array that is mechanically curved in the elevation direction or by using an acoustic lens, such as a silicon lens. A disadvantage of the 1D transducer is that an ultrasound image is created with a slice thickness that is optimal only in a range that is close to the focus of the lens.
2D arrays may be used to electronically control the focus in the out-of-plane dimension, allowing dynamic steering and focusing in any direction, including the elevation dimension. Unfortunately, this configuration greatly increases the number of channels in the system needed to drive the transducer as well as the number of electrical wires, such as coaxial cables, in the transducer cable. For example, for a square transducer having N elements along a side, N2 coaxial cables within the transducer cable and N2 channels for transmit and receive are needed.
Large numbers of wires or coaxial cables result in a transducer cable that is large and/or inflexible. The weight and inflexibility of the cable result in stress on the operator conducting the ultrasonic scanning, and may lead to repetitive stress injuries and fatigue. Also, the larger number of channels and coaxial cables increase the cost of both the system and transducer.
Several different transducers have been developed to address these problems. One type of transducer may be referred to as a 1.75D transducer. The 1.75D transducer may have N elements in azimuth and 2*M+1 elements in elevation (wherein M is a positive integer that is much smaller than N). This reduces the element count significantly compared to a 2D array with square elements. However, (2*M+1)*N coaxial cables and system channels are needed, which is at least three times the number needed for the 1D transducer.
Focusing delays of a 2D array scanning in the elevation plane are symmetrical along the elevation midline. A further reduction of coaxial cables may thus be obtained by electrically connecting elements that are symmetrically located along the midline. This type of array is typically referred to as a 1.5D array and is capable of dynamic elevation focusing on receive. After electrically connecting the elements, (M+1)*N coaxial cables and system channels are needed. This is lower than for the 1.75D array, but still at least twice the number needed for the 1D transducer.
A 1.25D array has also been used. The elevation aperture of the 1.25D array can be selected to increase with range, but the elevation focus of the aperture is static and determined by a mechanical lens. Typically, this type of array has a few programmable, such as 2-4, selections of apertures/foci, and the selection of the aperture/focus may be performed by electronic multiplexing circuits located in the transducer. The number of coaxial cables and system channels needed is no larger than for a 1D transducer. Switching between different selections is associated with artifacts, however, so dynamic focusing during reception is typically not feasible. This configuration may not be acceptable in scanning applications where frame rates are critical.
In another method, the elevation width of the array elements increases away from the midpoint of the array. The same number of system channels is required as the 1D array, while some degree of dynamic elevation focusing is possible on receive. However, the active area of the transducer aperture is reduced, resulting in a reduced far-field sensitivity compared to a 1.25D or 1.5D transducer.
Therefore, a need exists for a transducer capable of dynamic elevation focusing without sacrificing sensitivity or frame rate, and also without requiring a large increase in coax and system channel count compared to a 1D transducer.